Astroboffins think strangely porous boulders found on asteroid Ryugu may be the stuff of proto-planets

Asteroid Ryugu, the rock from which Japan’s Hayabusa2 probe brought home some samples, is home to highly porous boulders that might just have been the stuff of which planets were made. So says a new paper titled “Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body” published on May 24th …

[Chris G]

Pumice

Is produced from mostly volvanoes that have low silica lava which is low viscosity but has a high percentage of gases dissolved in it.

The gases coming out of solution are what provides the pressure for the eruption in the first place.

I can't see how a similar mechanism would function on a low gravity planetisimal, shirley it would more likely be an agglomeration of particles loosely held together to account for the porousity?

[UCAP]

Re: Pumice

If the planetesimal formed outside of the ice line (i.e. far enough from the proto-Sun for the gases in the accretion disk to freeze) then the particles would probably have had gasses either embedded in them or covering them. If the planetesimal subsequently moved closer to the Sun, the ices would have sublimed back to gas, leaving the porous structure behind.

[Tom 7]

Re: Pumice

But pumice is also fused together. Its whole structure is radically different in all but porosity. I know I've had some of it broken up on my chest with a sledgehammer. Wouldn't try that with fluffy sand for all the beer in the US!

[Pascal Monett]

Oh come on, if you had all the weak gnat's urine in the US you definitely be up for trying that.

[ThatOne]

Re: Pumice

> it would more likely be an agglomeration of particles loosely held together to account for the porousity?

Yes, of course, the pumice example is a misleading one, as both the creation process and the result are totally different, in all but the fun fact pumice very low density (it floats in water). Pumice is simply "solidified stone foam", and has nothing to do with what asteroids are made of.

Anyway.

I personally don't consider their findings surprising: The asteroid being clumped-together dust and having an extremely low gravity, I don't really see how it could be anything else than low-density. There is no gravity or rain to progressively compact the material, it's just dust and pebbles held together by the extremely low gravity of the asteroid.

The only way it could be high density, stone-like material is if the asteroid was a broken piece of a much bigger body, on which the high gravity and violent collisions had compacted and melted the dust to what we call "rock".

[Big_Boomer]

Could be dust particles accumulating due to static charge and eventually merging to form stronger bonds under localised micro-gravity, or could be solid matter that once contained various volatiles that sublimated leaving the solids behind. Sublimation tends to leave flow patterns behind though, so they need to be looking for those. Given the timescales involved I tend towards the static accumulation theory, but we do know that our planets have moved around quite a bit since their formation so no reason why asteroids wouldn't do the same, and that also backs up the sublimation theory.

[Tom 7]

I think one of the problems with trying to work out how our solar system formed is the tendency to go for simple and assume it was a fairly even disk of dust. Given our solar system almost certainly formed from two or more supernova shock-waves crashing into each other I think a nice homogeneous disk where the sun formed before the planets is probably one of the lowest probability scenarios there is,

[Pascal Monett]

Well obviously it couldn't have been a prefectly even disk of dust, else nothing would have had enough gravitational weight to attract anything else.

Besides that, we difinitely need to get away from the idea that the Universe is perfect. It's its imperfections that created it.

The rules that govern its existence, on the other hand, are perfect. We just need to find them out.

[M. Poolman]

I might be missing something here

but why the excitement that "this may be the stuff that planets were built from"? I thought the whole point of these missions was asteroids were already assumed to be just that. I'm not trying to diss what is obviously a stonkingly brilliant mission, just curious as to the way this particular result is being reported,

[ThatOne]

Re: I might be missing something here

> just curious as to the way this particular result is being reported

I suppose there was a certain intention to make a small announcement more "newsworthy".

Take the actual information, it just spells "much as expected, the asteroid seems to be very low-density". Obviously nobody but the specialists would care about that. But now sugarcoat it in "At last! We found the origins of our solar system!!!!" and you have something people will read...